Not sure about the version to use, but from my experience Yalp proved being an amazing Play store alternative as it allows downloading the .apk files, choosing the version manually, blacklisting auto-update for some applications only but not the others, using dummy Google accounts, and updating applications. It does not solve all issues with Google since you may still need Google services for some applications anyway, and it does not offer downloading .apk files if your phone is not compatible, but it's still a significant step forward. I really hope this application will survive when Google starts trying to block it.

Do you know how to relate the six digit number in the yalp manual download to the standard app version number?

I don't think there is a proper way to do that. Yalp help says that the numbers to enter in Yalp are just incremental based on the updates published to the store by the application maintainer, so if the version you want is the last one before the current version, you would need to let Yalp display the version number first (it shows the last one by default) and then subtract 1 to it. If you don't know how many versions have been published since your target version, then it gets a little more tricky.

I have not downgraded Here WeGo yet so I cannot help, but I'd be grateful if you report back the Yalp version number you had to get Here WeGo 2.0.11457 or 2.0.11539.

Yep you're right, I can't manage to download any previous version of Here WeGo manually. Not sure why. Maybe there is just no previous version available on the Play store, after all Yalp says that it is "sometimes" possible to download earlier versions, but it's not keeping copies of the files itself, everything is just downloaded from the Play store indirectly.

It may be worth asking on Yalp's github repository, the maintainer seems to be responsive and he also has some consideration for non Android-phones since he did include the Jolla C in the device-spoofing feature of his application.

In the meantime, maybe the best way is to find mirrors that keep version history.